The determination of a “flight envelope” for an aircraft is an important task typically undertaken by aeronautical engineers during the development and testing phases of aircraft design. Flight testing is performed to evaluate the characteristics of an aircraft and to determine a range of flight conditions in which an aircraft can be operated with safety. A particularly dangerous type of instability, and which is typically tested for, is flutter. Flutter results from a coupling between structural and aerodynamic forces. Flutter is an aerodynamically induced vibration, involving the coupling of two primary modes of vibration or a primary mode with a control rotation mode of vibration such that the aerodynamic forces overcome the structural damping forces to produce divergent oscillations.
In 1964, following the development, testing, and certification phases of the 20 Series Learjet, the aircraft was introduced into the market as the first true business class, jet propelled aircraft. In 1974, Lear followed the 20 Series Learjet with the introduction of the 30 Series Learjet, which included a number of refinements and enhancements to the 20 Series Learjet. However, one area of the aircraft's design that did not undergo modifications or refinements was the tail section.
Engineering analysis and modeling of aircraft which have structural dynamics very similar to Learjet's Series 20 and 30 aircraft have provided a predictive basis for a susceptibility of Learjet's Series 20 and 30 aircraft to tail flutter, during certain flight conditions within the certified flight envelope. When present, flutter manifests itself as a resonance that causes the horizontal stabilizer portion of the tail section to oscillate at around a 28 Hz frequency. Accordingly, there is a need for the preemptive reduction in the susceptibility of the horizontal stabilizer to flutter within the Series 20 and Series 30.